1. (Industrial useful field)
This invention relates to a lead-acid battery plate making use of copper for a grid and its manufacturing method.
2. (Prior art and its problem)
Generally, lead alloy has been used for a grid for a lead-acid battery plate.
In a lead-acid battery making use of a grid comprising lead alloy for its plate, however, an electric resistance of the lead alloy is as high as about 21 .mu..OMEGA.-cm, so that an ohmic resistance polarization due to a grid resistance becomes large to cause a drop of discharge voltage when the battery is discharged with a large current. Further, a specific gravity of the lead alloy is as large as 11 so that a weight efficiency is low. Moreover, when a height of the plate increases, a grid resistance gives a large influence on battery characteristics, so that only an upper part of plate provided with a plate lug is concerned with charge and discharge reactions and a lower part thereof is not concerned with them. For this reason, the grid made of lead alloy has not been suitable for use in a tall plate composing a large lead-acid battery.
Therefore, the use of copper having a better electric conductivity than lead alloy was proposed, and a plate as illustrated by FIG. 7 making use of an expanded metal comprising copper as the grid has been adopted. In FIG. 7, 1 is a plate lug comprising lead alloy, 2 3 is a grid made of the expanded metal comprising copper on which lead alloy or lead tin alloy is electroplated, is a negative active material, and 4 is a lower connection body. Here, the reason why the lead alloy or lead-tin alloy is electroplated on the grid 2 is that the copper dissolves in electrolyte of the battery to cause an increase in self-discharge, a reduction in capacity, and a lowering of charging efficiency etc.
However, since a lead layer is soft, a side layer of the grid is especially apt to be scratched in pasting and formation of later process and the copper is exposed so that the foregoing problem has occurred. Further, since a side of the grid is rough, the active material 3 adhering to its side falls down during operation of the battery and becomes sponge-like shape to cause short-circuit so that a service life of the battery has been exhausted in a short period. There is a method where the active material 3 adhering to its side is removed in a later process in order to avoid this short-circuit, however, a number of process has been increased and the layer has been scratched to expose the copper in this method.
The plate as illustrated by FIG. 7 has conventionally been manufactured in such a way that the lead alloy or lead-tin alloy is electroplated on the copper expanded metal and then the plate lug 1 is connected thereto by casting.
Since the layer covering on the copper has a melting point lower than a melting temperature of the lead alloy to be cast, the layer around a boundary surface between the grid 2 and the plate lug 1 has been molten to become thin or peeled off, in an extremely case, to cause the copper to be exposed during the casting. In order to prevent the copper from being exposed, a double electroplating layer structure has been employed: the first electroplating layer of lead-tin alloy or tin alloy which is easy to be alloyed with copper, and the second electroplating layer of lead. However, this method has not been able to completely prevent the layer from peeling off.
Further, there is such a method that a casting temperature is strictly controlled so as to carry out the casting as low temperature as possible. In this method, however, a failure in forming the plate lug 1 has occurred and fitting the connection between the grid 2 and the plate lug 1 has become weak so that a grid resistance has increased.